1. Field of Invention
This invention relates to a polyblend resin having excellent resistance to hydrolysis, and more particularly it relates to a polyblend composition essentially consisting of a mixture of polyvinyl chloride and polyester-urethane resin, a lead stabilizer and small quantities of antimony trioxide and one or more kinds of polyhydric alcohol selected from the group consisting of sorbitol, mannitol, pentaerythritol, erythritol, arabitol, xylitol and dulcitol.
It is another object of this invention to provide a method for producing a molding from said type of a polyblend composition by adding, in advance to molding, antimony trioxide and one or more kinds of polyhydric alcohols selected from said group in a pre-determined amount to a mixture of a vinyl chloride resin and polyester-urethane resin and then molding the polymer mixture.
It is further object of this invention to provide a molding product obtained from the above mentioned resin composition, which has excellent resistance to hydrolysis.
2. Description of Prior Arts
Lead stabilizers such as tribasic lead sulfate, tribasic lead phosphate, dibasic lead stearate, etc., are commonly used as thermal stabilizers for thermoplastic molding of polyvinyl chloride resins which are widely employed for molding of various kinds of industrial and general materials such as electric wires, leather articles, hoses, etc. It therefore easily comes to mind to use such lead stabilizer for attaining thermal stability in molding of resin blends consisting of polyvinyl chloride resin and polyurethane resin, particularly polyester-urethane resin, but the results of experiments revealed that when a polyblend resin molding formed by adding a lead stabilizer is kept in hot water or in a moist and hot atmosphere, the polyester-urethane resin component undergoes hydrolysis at a high rate, and in some extreme cases, polyester-urethane with number-average molecular weight of several ten thousands might be decomposed into a syrupy oligomer with a molecular weight of several hundreds and could exude to the molding surface or secede from the molding.
For these reasons, it is inexpedient to put said polyblends to practical use under strict conditions such as abovementioned. Even though there are actually few occasions of use under such circumstances, the above-said undesirable effect is gradually accumulated to cause a fatal damage in long-term use if such polyblends are used in articles which are required to have extremely long service life, such as for example electric wires. This holds true with other kinds of articles for different uses although the affected degree may differ from one article to the other.
It is already known that polyester-urethane resins, that is, polyurethane resins containing ester groups, are very poor in resistance to hydrolysis in hot water or under hot and humid conditions, and for the solution of this problem, it has been proposed to add carbodiimide in the composition. (See, for example, Japanese Patent Publication No. 23311/1963; W. Kellert: Journal of the IRI, Vol. 2, No. 1, Jan./Feb., 1968, pp. 26-28; and E. Muller: Die Angewandte Makromolekular Chemie, 14 (1970), pp. 75-86 (Nr. 203)). However, addition of carbodiimide to the blends of polyester-urethane resin and polyvinyl chloride resin does not provide satisfactory improvement of the hydrolytic resistance of the blends. For instance, in the tests conducted by adding 2,2'-dimethyldiphenylcarbodiimide in an amount ranging from 0 to 5 weight parts per 100 weight parts of the polyblend composition, it was found that hydrolysis took place to an excessive degree as a result of 7-day immersion in boiling water as described later in more detail in comparison with reference examples. This could be easily determined by macroscopic observation or by finger touch of surface tackiness. It is to be also noted that carbodiimide is relatively costly, and its use as an additive is not recommendable from the viewpoint of cost performance.